Strain detector

ABSTRACT

A strain detector includes an insulating substrate, a power electrode, output electrodes and a ground electrode which are disposed over the insulating substrate, a strain-resistance element provided over the insulating substrate and coupled to the power electrode, the output electrodes and the ground electrode, a protective layer over the strain-resistance element, a frame ground electrode. The insulating substrate includes a stainless steel board, a protective coat on the stainless steel board, and an insulating layer on the protective coat. The frame ground electrode is provided over the stainless steel board and electrically coupled to the stainless board.

This application is a Continuation-in-part of U.S. Ser. No. 09/749,756,filed Dec. 28, 2000 now U.S. Pat. No. 6,761,073.

FIELD OF THE INVENTION

The present invention relates to a strain detector for detecting astrain resulting from a load.

BACKGROUND OF THE INVENTION

Japanese Patent Laid Open Publication No. 8-87375 discloses aconventional strain detector. The conventional strain detector will bedescribed with reference to drawings hereinafter. FIG. 9 is a top viewof the conventional strain detector, and FIG. 10 is a cross sectionalside view of the detector.

In FIG. 9 and FIG. 10, insulating substrate 1 made of elastic materialis formed by disposing stick member 2 and insulating layer 3 thereon.Four strain-resistance elements 4 are disposed over insulating substrate1. Strain-resistance elements 4 are electrically coupled to a pair ofpower electrodes 5, a pair of output electrodes 6, and a pair of ground(GND) electrodes 7 to form a bridge circuit. Protective layer 8 made ofresins covers elements 4, power electrodes 5, the pair of outputelectrodes 6, the pair of GND electrodes 7 and the rest of insulatingsubstrate 1

The operation of the above conventional strain detector will bedescribed hereinafter.

When a shearing load is applied on the general-center position of thetop of insulating substrate 1, a bending moment occurs in insulatingsubstrate 1 via the shearing load and also occurs in the fourstrain-resistance elements 4 disposed over substrate 1. A resistance ofstrain-resistance elements 4 changes by the bending moment resulting inelements 4. A change of the resistance is supplied from the pair ofoutput electrodes 6 to an external measuring device (not shown), andthen the load on substrate 1 is measured.

In the conventional strain detector, only protective layer 8 made ofresins is disposed over insulating substrate 1, the pair of powerelectrodes 5, the pair of output electrodes 6 and the pair of GNDelectrodes 7. Protective layer 8 made of resins absorbs water little bylittle. Therefore, when the strain detector is used for a long time inan atmosphere of high humidity, the water reaches strain-resistanceelements 4 and the resistance of strain-resistance elements 4fluctuates.

SUMMARY OF THE INVENTION

A strain detector includes an insulating substrate, a power electrode,output electrodes and a ground electrode which are disposed over theinsulating substrate, a strain-resistance element provided over theinsulating substrate and coupled to the power electrode, the outputelectrodes and the ground electrode, a protective layer over thestrain-resistance element, a frame ground electrode. The insulatingsubstrate includes a stainless steel board, a protective coat on thestainless steel board, and an insulating layer on the protective coat.The frame ground electrode is provided over the stainless steel boardand electrically coupled to the stainless board.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a strain detector in accordance with ExemplaryEmbodiment 1 of the present invention.

FIG. 2 is a cross sectional side view of the strain detector inaccordance with Embodiment 1.

FIG. 3 is a cross sectional view of the strain detector in accordancewith Embodiment 1.

FIG. 4 is a top view of the strain detector in accordance withEmbodiment 1.

FIG. 5 is a top view of the strain detector in accordance withEmbodiment 1.

FIG. 6 shows a distribution of a stress occurring in a rectangularinsulating substrate in accordance with Embodiment 1.

FIG. 7 shows a distribution of a stress occurring in the insulatingsubstrate of the strain detector having a constriction section in theinsulating substrate in accordance with Embodiment 1.

FIG. 8 is a cross sectional view of a strain detector in accordance withExemplary Embodiment 2 of the invention.

FIG. 9 is a top view of a conventional strain detector.

FIG. 10 is a cross sectional side view of the conventional straindetector.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Exemplary Embodiment 1

FIG. 1 is a top view of a strain detector in accordance with ExemplaryEmbodiment 1 of the present invention. FIG. 2 is a cross sectional sideview of the strain detector at a position of the strain-resistanceelements disposed therein. FIG. 3 is a cross sectional side view of thestrain detector at a position of electrodes disposed therein. FIG. 4 isa top view showing a state of a conductive adhesive disposed in a slitsection of the insulating substrate.

In FIG. 1 through FIG. 4, insulating substrate 11 is made of elasticmaterials and includes stainless steel board 12 containing aluminum,protective coat 13 made of alumina disposed over board 12, andinsulating layer 13 a made of glass disposed over coat 13. According toembodiment 1, protective coat 13 is made of alumina, i.e., insulatingmaterial, but has a thickness of about 0.2 μm and a small resistanceranging from 2Ω to 30Ω, hence functioning as a conductive element. Powerelectrode 14, a pair of output electrodes 15, and ground (GND) electrode16, which are made of silver, and strain-resistance elements 17 areelectrically coupled to each other via circuit pattern 18 to form abridge circuit. Moreover, temperature-characteristic adjusting resistor19 used as a temperature compensation element is disposed over substrate11. One end of resistor 19 is electrically coupled to GND electrode 16and the other end is coupled to strain-resistance elements 17 via a pairof resistance-measuring electrodes 20. Capacitor 22 is coupled viacircuit pattern 18 in parallel to static-electricity-dischargingresistor 23 between GND electrode 16 on insulating layer 13 a and frameground (GND) electrode 21 on protective coat 13 and over board 12. Slit24 is disposed in circuit pattern 18 on insulating substrate 11 so thatslit 24 cuts off circuit pattern 18 partially, and a pair of slitelectrodes 25 which are made of silver and electrically coupled tocircuit pattern 18 is disposed at a position of slit 24. Conductiveadhesive 26 electrically connects the pair of slit electrodes 25 to eachother. First plate layer 27 made of nickel is disposed over powerelectrode 14, the pair of output electrodes 15, and GND electrode 16.Second plate layer 28 made of solder is disposed over first plate layer27. Four strain-resistance elements 17 are arranged in pairs overinsulating substrate 11, moreover, constriction section 11 a is disposedbetween elements 17 of each pair in substrate 11. First protective layer29 made of glass covers insulating substrate 11, power electrode 14, thepair of output electrodes 15, GND electrode 16, andtemperature-characteristic adjusting resistor 19. Second protectivelayer 30 made of resins or glass covers first protective layer 29.Capacitors 22 are coupled between power electrode 14 and GND electrode16 and coupled between output electrode 15 and GND electrode 16respectively.

A method of manufacturing the above detector will be described below.

First, insulating substrate 11 is formed by previously printing glasspaste over stainless steel board 12 containing aluminum and by baking atabout 850° C. for about 10 minutes.

Then, metal-glaze based paste is printed where strain-resistanceelements 17 and static-electricity-discharging resistor 23 are to bedisposed over substrate 11, and dried at about 130° C. for about 10minutes.

Then, thermistor-resistance-paste is printed at a position wheretemperature-characteristic-adjusting resistor 19 is to be disposed overinsulating substrate 11. And then, four resistance elements 17,static-electricity-charging resistor 23 and resistor 19 are formed bybaking at about 850° C. for 10 minutes.

Then, power electrode 14, the pair of output electrodes 15, GNDelectrode 16, resistance-measuring electrodes 20, circuit pattern 18,frame GND electrode 21 and slit electrode 25 are formed by printingsilver paste over insulating substrate 11 and by baking at about 600° C.for 10 minutes.

At this time, stainless steel board 12 containing aluminum is notoxidized by the baking because of the high heat stability of protectivecoat 13 made of alumina formed over board 12. Consequently, elasticproperties of substrate 11 becomes stable, and then the output of thestrain detector become stable.

Then, first protective layer 29 is formed by printing a paste of glassover substrate 11 but not over power electrode 14, the pair of outputelectrodes 15, GND electrode 16, resistance-measuring electrode 20 andslit electrode 25, and then by baking at about 600° C. for 10 minutes.

Then, the pair of temperature-characteristic-adjusting resistors 19 aretrimmed so that the pair of output electrodes 15 can output the sameamount of changes according to a change of a temperature surroundingsubstrate 11 under the condition that a voltage is applied to powerelectrode 14 coupled to a power supply while GND electrode 16 isgrounded.

Then, second protective layer 30 is formed by printing a paste of resinsor glass over first protective layer 29 disposed over substrate 11 andby baking at about 200° C. for 30 minutes.

Then, first plate layer 27 made of nickel is formed over power electrode14, the pair of output electrodes 15, and GND electrode 16. Then, secondplate layer 28 made of solder is formed over first plate layer 27. Slit24 breaks circuit pattern 18 to form slit electrodes 25, and further,conductive adhesive 26 is disposed at slit 24 to connect slit electrodes25 each other, which are provided at a portion of circuit pattern 18broken by slit 24. Namely, first plate layer 27 and second plate layer28 are formed over power electrode 14, the pair of output electrodes 15,and GND electrode 16 under the condition of electrically disconnectingstainless steel board 12 from power electrode 14, the pair of outputelectrodes 15, and GND electrode 16. Therefore, partially exposedstainless steel board 12 is not plated. As each electrode becomesstable, the amount of plating in each electrode become stable.Conductive members such as jumpers can be employed instead of theconductive adhesive.

Then, conductive adhesive 26 is painted over slit 24 and the end ofcircuit pattern 18 adjacent to slit 24 over substrate 11.

Then, capacitor 22, for coupling both circuit patterns, coupled to powerelectrode 14 and GND electrode 16, respectively, is mounted and issoldered to circuit pattern 18.

Also, capacitor 22 for coupling both circuit patterns of outputelectrodes 15 and GND electrode 16 is mounted and soldered to circuitpattern 18.

Then, capacitor 22 and discharging resistor 23 for coupling both circuitpattern 18, connected to frame GND electrode 21, and GND electrode 16,are mounted and soldered to circuit pattern 18.

The operation of the strain detector assembled above will be describedbelow.

When a shearing load is applied on the general-center position ofinsulating substrate 11, a strain occurs on the surface of substrate 11by the load, and also a strain occurs in the four strain-resistanceelements 17. When the strain occurs in strain-resistance elements 17,resistance of each element 17 changes. The change of the resistance issupplied to a measuring device (not shown), such as an externalcomputer, from the pair of output electrodes 15, and then, the load onsubstrate 11 is determined.

In this strain detector, second protective layer 30 made of resins orglass covers first protective layer 29 made of glass. Therefore, evenwhen water flows through second protective layer 30 because of using thedetector in a high humidity atmosphere for a long time, the water doesnot penetrate through first protective layer 29 made of glass.Consequently, as water does not reach strain-resistance elements 17, theresistance of strain-resistance elements 17 does not fluctuate, and thestrain detector from which can be obtained a stable output at all timesis provided.

If second protective layer 30 is made of resins, it is baked at arelatively lower temperature of about 200° C. Therefore, the resistanceof strain-resistance elements 17 andtemperature-characteristic-adjusting resistor 19 hardly change whilesecond protective layer 30 is being baked.

In this strain detector, temperature-characteristic-adjusting resistor19 is disposed over insulating substrate 11; however, thermistor 31 maybe disposed over substrate 11 instead of resistor 19. When thermistor 31is disposed, it can measure a temperature of substrate 11. Even if theresistance of strain-resistance elements 17 changes because of using thedetector in a temperature-changing atmosphere, the measuring device suchas a computer (not shown) can compensate for the change of theresistance of elements 17. Consequently, the load on the strain detectorcan be exactly detected if a compensation-value calculator 31A, such asan IC chip, for the compensation is mounted on the insulating substrate11. Wiring to an external measuring device becomes simple, and a load onthe measuring device is reduced.

The thermistor 31 is formed over substrate 11 in the same way asresistor 19. Or, as shown in FIG. 5, thermistor 31 can be mounted onsubstrate 11 in the same way as capacitor 22. In FIG. 5, thermistor 31is coupled to power electrode 14; however, thermistor 31 can be coupledto GND electrode 16 or floated by itself.

FIG. 6 shows an analyzed strain on insulating substrate 11. Whensubstrate 11 is rectangular like the conventional strain detector,bending strain concentrates at the end of substrate 11. The elasticcoefficient of substrate 11 therefore deteriorates. In the straindetector of Embodiment 1, constriction section 11 a is disposed betweenstrain-resistance elements 17 of each pair on substrate 11. Therefore,when a load is applied on the general-center position of the straindetector, a strain on the surface of substrate 11 is spread towardconstriction section 11 a from the end of substrate 11 as shown in FIG.7. The strain does not concentrate accordingly into the end of substrate11, and strain-resistance elements 17 can be placed at a wide area onsubstrate 11. And then, an assembling efficiency is improved.

For the case that static electricity more than 5 kV is applied to GNDelectrode 16 by touching it with a hand, the conventional straindetector requires a structure such that static electricity cannot beapplied to the GND electrode. That is because the insulating layer overthe substrate may break down. In the strain detector of Embodiment 1,frame GND electrode 21, which is disposed on protective coat 13, theconductive element, over stainless steel board 12 and electricallycoupled to stainless steel board 12, is electrically coupled to GNDelectrode 16. Consequently, static electricity runs through frame GNDelectrode 21 and to the GND electrode 16 via stainless steel board 12.Even when static electricity is applied to the GND electrode 16,insulating layer 13 a can be prevented from breaking down.

In the strain detector of Embodiment 1, a discharge element, whichincludes capacitor 22 and static-electricity-discharging resistor 23connected in parallel, is coupled between frame GND electrode 21 and GNDelectrode 16. Therefore, even if static electricity is applied to GNDelectrode 16, capacitor 22 absorbs the electricity. As a result, thestatic electricity is controlled within a low voltage, and insulatinglayer 13 a can be prevented from breaking down. When resistor 23discharges the accumulated electrical charge, GND electrode 16 and frameGND electrode 21 reach the same potential. As stainless steel board 12is not directly coupled to GND electrode 16, the potential of GNDelectrode 16 does not fluctuate, and output signals from the outputelectrodes 15 become stable.

In the strain detector of Embodiment 1, when the static electricity isapplied to power electrode 14 or output electrodes 15, for example, bytouching with a hand, capacitors 22 absorb an electric charge of theelectricity because capacitors 22 are coupled between power electrode 14and GND electrode 16, and between each of the pair of output electrodes15 and GND electrode 16, respectively. Therefore, as the staticelectricity is controlled within a low voltage, and excessive currentdoes not run through strain-resistance element 17, the resistance ofstrain-resistance elements 17 becomes stable.

According to Embodiment 1, the strain detector having thetemperature-characteristic-adjusting resistor, the slit, the conductivemembers or the static-electricity-discharging element is described asshown in FIG. 1. The same factors can be additively disposed in thedetector having a thermistor as shown in FIG. 5.

In the strain detector of Embodiment 1, first plate layer 27 made ofnickel is formed over power electrode 14, GND electrode 16, and outputelectrodes 15, and also, second plate layer 28 made of solder is formedover first plate layer 27. Therefore, silver does not move from eachelectrode to second plate layer 28. As a result, the electricconnections between each electrode and a terminal of an external devicebecomes further reliable.

Exemplary Embodiment 2

FIG. 8 is a cross sectional view of a strain detector in accordance withExemplary Embodiment 2 of the invention.

In a strain detector shown in FIG. 3 according to Embodiment 1, Frameground (GND) electrode 21 is provided on protective coat 13, aconductive element, on stainless steel board 12, and frame GND electrodeis electrically coupled to stainless steel board 12.

In the strain detector shown in FIG. 8 according to FIG. 8, insulatingsubstrate 111 is made of elastic material and includes stainless steelboard 12 containing aluminum, protective coat 113 made of alumina onboard 12, and insulating layer 13 a made of glass on protective coat113. Frame GND electrode 121 functioning similarly to frame GNDelectrode 21 according to Embodiment 1 is provided on stainless steelboard 12. Protective coat 113 is provided on a portion of stainlesssteel board 12 other than a portion of board 12 on which frame GNDelectrode 121 is provided.

For the case that static electricity more than 5 kV is applied to GNDelectrode 16 by touching it with a hand, in the strain detector ofEmbodiment 2, frame GND electrode 121 on stainless steel board 12 andelectrically coupled to board 12 is electrically coupled to GNDelectrode 16. Consequently, static electricity runs through frame GNDelectrode 121 and to the GND electrode 16 via stainless steel board 12.Even when static electricity is applied to the GND electrode 16,insulating layer 13 a can be prevented from being broken down.

In the strain detector according to Embodiment 2, frame GND electrode121 contacts stainless steel board 12 directly. Therefore, a resistancebetween frame GND electrode 121 and stainless steel board 12 is smallerthan a resistance between frame GND electrode 21 and stainless steelboard 12 according to Embodiment 1 which are provided so that protectivecoat 13 is located between electrode 21 and board 12. The straindetector according to Embodiment 2 includes insulating layer 13 aprotected from static electricity more. Further, protective coat can bethick and allows stainless steel board 12 to be strong.

1. A strain detector comprising: an insulating substrate including astainless steel board, a protective coat on said stainless steel board,and an insulating layer on said protective coat; a power electrode,output electrodes and a ground electrode disposed over said insulatingsubstrate; a first strain-resistance element provided over saidinsulating substrate and coupled to said power electrode, said outputelectrodes and said ground electrode; a first protective layer over saidfirst strain-resistance element; a frame ground electrode provided oversaid stainless steel board and electrically coupled to said stainlessboard; a circuit pattern disposed over said insulating substrate forconnecting said frame ground electrode to said ground electrode, saidcircuit pattern having a slit formed therein to break said circuitpattern; and a conductive member provided at said slit for connecting aportion of said circuit pattern broken by said slit.
 2. The straindetector according to claim 1, wherein said protective coat comprisesalumina.
 3. The strain detector according to claim 1, wherein saidprotective coat comprises a conductive element, and wherein said frameground electrode is provided on said protective coat.
 4. The straindetector according to claim 1, wherein said frame ground electrode isprovided on said stainless steel board.
 5. The strain detector accordingto claim 1, wherein said insulating layer comprises glass.
 6. The straindetector according to claim 1, wherein said insulating substratecomprises elastic material.
 7. The strain detector according to claim 1,wherein said first protective layer comprises glass.
 8. The straindetector according to claim 1, further comprising atemperature-characteristic-compensation element disposed over saidinsulating substrate.
 9. The strain detector according to claim 8,wherein said temperature-compensation element comprises atemperature-characteristic-adjusting resistor coupled to said firststrain-resistance element.
 10. The strain detector according to claim 8,wherein said temperature-characteristic-compensation element comprises athermistor.
 11. The strain detector according to claim 8, furthercomprising a compensation-value calculator for compensating an output ofsaid first strain-resistance element based on an output of saidtemperature-characteristic-compensation element, said compensation-valuecalculator disposed over said insulating substrate.
 12. The straindetector according to claim 1, further comprising: a capacitor coupledbetween said power electrode and said ground electrode; and capacitorscoupled between said output electrodes and said ground electrode,respectively.
 13. The strain detector according to claim 1, furthercomprising: a first plate layer disposed over said power electrode, saidground electrode, and said output electrodes; and a second plate layerdisposed over said first plate layer.
 14. The strain detector accordingto claim 13, wherein said first plate layer comprises nickel, and saidsecond plate layer comprises solder.
 15. The strain detector accordingto claim 1, further comprising a second strain-resistance elementprovided over said insulating substrate and coupled to said powerelectrode, said output electrodes and said ground electrode, whereinsaid insulating substrate has a constriction section provided betweensaid first strain-resistance element and said second strain-resistanceelement.
 16. The strain detector according to claim 1, wherein saidframe ground electrode is electrically coupled to said ground electrode.17. The strain detector according to claim 16, further comprising astatic-electricity-discharging element coupled between said frame groundelectrode and said ground electrode.
 18. The strain detector accordingto claim 17, wherein said static-electricity-discharging elementcomprises a static-electricity-discharging resistor and a capacitorcoupled to said static-electricity-discharging resistor connected inparallel to said static-electricity-discharging resistor.
 19. The straindetector according to claim 1, further comprising a second protectivelayer on said first protective layer.
 20. The strain detector accordingto claim 1, wherein said conductive member comprises a conductiveadhesive.
 21. The strain detector according to claim 19, wherein saidsecond protective layer comprises at least one of glass and resin.